1. Field of the Invention
This invention relates to a control device of a crane having a jib or a boom which can be raised for use in winding up a hanging load in a vertical direction to release it from a ground and more particularly a control device of a vertical releasing for preventing the load from being vibrated during its releasing from the ground.
2. Description of the Prior Art
As already disclosed in the specification of Jap.Pat.Publn. No.Sho 59-26599, in case that a load is wound up by a winding-up rope hung from an extreme end of a jib in a crane provided with the jib which can be raised freely, the extreme end of the jib is coincided with a vertical line passing through a center of gravity of the hanging load. However, after this operation, as the winding-up rope is wound up, the jib or the like is flexed, resulting in that the extreme end of the jib is displaced in a forward direction from upon the vertical line. In this way, as only the winding-up rope is wound up while the extreme end of the jib or the like is being displaced, the hanging load is moved in a foreward direction just when the hanging load is released from the ground (the load is released from the ground), resulting in forming a load oscillation.
In the specification of the aforesaid Jap.Pat.Publn.No. Sho 59-26599 is disclosed the following control means in order to prevent the load from being oscillated when the load is released from the ground as described above. That is, an energizing torque instruction signal varying in a step-wise manner as a time elapses is outputted from a winding-up energization torque instruction device on the basis of a difference between a speed instruction value outputted from the winding-up instruction device and a speed sensing value of a winding-up electric motor and then a winding-up speed of the winding-up rope is feed back controlled in response to the instruction signal. In turn, an amount of flexing of the jib is instructed from the flexing amount instruction device in response to a momentum of the jib, a raising speed is outputted from the jib raising amount instruction device in response to the flexing amount and the aforesaid winding-up speed instruction signal, and the jib raising speed is feed back controlled in response to a difference between the speed instruction and a rotational speed sensing value of the jib raising electric motor. In this way, a displacement of the extreme end in a forward direction is corrected by raising the jib.
The aforesaid control means has the following problems.
A. Since a driving torque instruction for winding-up the hanging load is increased in a stepwise manner, a hanging load winding-up speed is substantially varied when the operating steps are varied and a transient oscillation may easily be produced. It takes much time until the releasing operation is finished as compared with the case in which the hanging load winding-up speed is continuously increased.
B. In order to minimize a load oscillation during the releasing of the load from the ground, it is necessary to control a position of the extreme end of the jib in such a way as the position of the extreme end of the jib always occupies just above the vertical line of the hanging hook. However, in case of the aforesaid control means, a retracting speed of the jib is correspondingly controlled in response to the winding-up speed of the hanging load, an accurate position control of the extreme end of the jib may not be carried out and there is no assurance that the extreme end of the jib always lies just above the hanging load just when the load is released, resulting in that is it hard to make a positive prevention of the load oscillation.
C. Although the aforesaid control means calculates a flexing amount of the extreme end of the jib in response to a momentum of the jib and controls a raising speed of the jib in response to the flexing amount, the flexing amount is not determined only with the momentum. That is, even with the same momentum, the flexing amount is varied in response to an initial jib angle or a jib length or the like. For example, in case that the jib initial angle is low, if the jib is raised, a tension applied to the winding-up rope is rapidly increased due to a large displacement amount toward the vertical upward direction as compared with a horizontal displacement amount of the extreme end of the jib, resulting in that the hanging load is released before the forward horizontal displacement amount of the extreme end of the jib is corrected. In order to prevent this phenomenon, it is necessary to raise the jib while the winding-up rope is wound down in such a way that a rate of variation of a rope tension force is kept at a specified limited value. However, in case of the aforesaid control means, the controlling operation is carried out in reference to the winding up of the winding-up rope, so that the winding-down control may not be carried out.
In turn, in the specification of Jap.Pat.Laid-Open No.Sho 62-191393 is disclosed another means differing from the aforesaid control means. The control means is operated such that in order to cause the position of the extreme end of the boom to be placed just above the vertical location of the hanging load through a single operation of the boom raising, a moving speed v (v=L.cos .theta...OMEGA.) of the extreme end of the boom in a vertical direction produced under the boom raising operation is calculated in response to a boom length L, a cosine value of a boom raising angle .theta. and a boom raising angular speed .OMEGA., the speed v is applied as a moving speed instruction value of the winding-up rope, and the moving speed of the winding-up rope is controlled by a servo control device in such a way as a difference between the instruction value and the detected value of the moving speed of the winding-up rope becomes zero.
In case of this control means, in order to correct a position of the extreme end of the boom displaced by the raising operation of the boom, the moving speed of the winding-up rope is controlled so as to keep a rate of variation of a tension of the rope constant. A mere control of the moving speed of the winding-up rope is difficult to perform an accurate position control of the extreme end of the boom, it may happen that the hanging load is released from the ground while the extreme end of the boom is being displaced from the vertical line passing through a center of gravity of the hanging load and so it is difficult to make a positive prevention of the load oscillation.
In the specification of Jap.Pat.Laid-Open No.Sho 61-211296 is disclosed means for controlling a winding-up speed of the winding-up rope and a jib raising speed in order to prevent a load oscillation during a widing-up of the winding-up rope in such a way as an oscillation angle of the winding-up rope in respect to the vertical line is detected and its oscillation angle becomes a set value. However, with such a control means, it is difficult to make an accurate detection of the oscillation angle of the winding-up rope. Due to this fact, a control over each of the speeds described above becomes inaccurate and the load oscillation may not be positively prevented.
Each of the aforesaid well-known control means is applied to a so-called round hanging object releasing operation in which the extreme end of the jib at its initial state, a hanging element for the extreme end of the winding-up roper and a center of gravity of the hanging load are coincided to each other. In case of hanging the round load, it is sufficient that only the displacement (a forward falling amount) of the extreme end of the jib caused by the flexing of the jib as the hanging load applied to the winding-up rope is increased is corrected.
To the contrary, in case that a long hanging load such as a column or a pile or the like is raised from its fallen (lowered) state to its vertical state to release from the ground, it is needed to perform a correction for displacing the extreme end of the jib by an amount corresponding to a length of the handing load in addition to a correction of the displacement of the extreme end of the jib in order to prevent a displacement of position of the hanging load or a load oscillation. In some cases, a correction in a swivelling direction is also required and then a three-dimensional correction of position is needed.
Each of the well-known control means may not perform an automatic correction of position. Due to this fact, in case that the long hanging load is to be released from the ground, it is actually performed that an operator raises the jib through his manual operation, winds up or winds down the winding-up rope in response to a signal from a load hanging person or in reliance upon his experience and operational guess-feeling while looking at the hanging load (in some cases, also its swivelling action).
In case that the long hanging load is mounted in a longitudinal (an aft and fro) direction as viewed from the operator, it is needed to perform a substantial correction of the position of the extreme end of the jib in its aft and fro direction as the hanging load is changed from its fallen state to its raising state. In this case, it is difficult for the operator to get a degree of inclination of the aft and fro directions of the hanging load and the jib, an efficiency in operation is poor under the aforesaid manual operation, a fine adjustment of the position of the extreme end of the jib in an aft and fro direction is difficult and so an accurate control of the position may not be attained. Due to this fact, a position of the hanging load is sometimes displaced before releasing it from the ground or the hanging load is widely oscillated forwardly and rearwardly just when the releasing from the ground is carried out. However, a displacement in position of a certain hanging load is not always allowed and a substantial oscillation of the hanging load is dangerous.